One of the major obstacles preventing the commercialization of tar sand and oil shale is the high cost associated with mining and processing the solid feed material, upgrading raw crude oil, and refining upgraded crude oil to produce salable products. In order to commercialize tar sand and shale oil recovery while reducing the financial risks, new processing technologies must be developed to obtain the following goals:
Much greater oil yield;
Products that require a minimum of upgrading to produce a finished, marketable fuel;
Advanced fuel feedstocks;
Asphalt binders;
Many processes have been developed to recover oil from tar sand and oil shale, but so far none of these processes has accomplished all of the above objectives.
In 1953, Jensen et al. in "Thermal Solution and Hydrogenation of Green River Oil Shale," Bulletin 533 of the U.S. Bureau of Mines, showed that when oil shale was heated in the presence of a solvent and souluble conversion products were extracted with the solvent rather than distilled from the shale, the rate of kerogen decomposition was significantly increased, and a higher percentage of kerogen was converted to volatile products. This study was performed by treating a slurry of shale oil and pulverized oil shale at about 300 psi pressure and temperatures ranging from 750.degree. to 850.degree. F., for various reaction times. Despite the advantages mentioned above, this thermal-solution process was not considered to be a practical shale oil production process because of the high processing costs associated with the grinding of the shale, the high boiling range of the oil produced, the cracking accompanied by polymerization and condensation of shale oil solvent, and the difficulty of separating the solvent and product oil from the spent shale.
Some of the disadvantages of the thermal-solution process described above may be overcome by using larger size oil shale and a recycle shale oil, and by continuously withdrawing oil vapor from the retort. In this case, the bitumen may be continuously removed from the shale particles by retorting the oil shale in the presence of a shale oil to increase the oil yield. Additionally, the heavy oil absorbed in the spent shale may be recovered by heating the shale at temperatures slightly greater than the retorting temperature in the absence of recycled oil. Recovered heavy oil may then be recycled to the retort.
High-density fuels are liquid hydrocarbon mixtures, mostly made up of cyclic saturated hydrocarbons called cycloparaffins. Since these fuels provide more operating miles between refuelings for volume-limited aircraft and tactical ground equipment, these fuels are particularly attractive for military use. These high-density fuels are made by hydrogenating aromatic oils to make cycloparaffins.
Tar sand and eastern oil shale contain a high percentage of aromatic hydrocarbons. Consequently, it is possible to produce oil product containing high concentrations of aromatic compounds from tar sand or eastern oil shale. The aromatic compounds can be produced by retorting the tar sand or eastern oil shales at very high temperatures, or in the presence of heavy oil solvent. A number of solvent extraction processes have been developed for recovering oil from tar sand and oil shale. All of the existing solvent extraction processes for oil shale require high pressure. The solvent extraction processes for tar sand produce the bitumen which requires an extensive treatment to be a marketable fuel. As a result, the operation and capital costs are too high for existing solvent extraction processes to be competitive with natural crude oil.
Reed, Jr., in U.S. Pat. No. 3,939,057, discloses a process for obtaining petroleum products from oil shale by retorting crushed oil shale admixed with a small quantity of crushed coal in an indirectly heated, rotary calciner. The oil free shale is fed to a furnace where it is burned to produce the heat for retorting and preheating the shale.
Mitchell, in U.S. Pat. No. 4,401,551, discloses a method for extracting oil from shale comprising contacting previously recovered bitumen with bituminous sand and separating the resulting liquids from the resulting solids. This process is conducted in a vertically oriented bed.
Williams et al., in U.S. Pat. No. 4,108,760, disclose a process for extracting oil shales and tar sands by heating the shale in the presence of an extractant gas at a temperature ranging from 0.degree. to 550.degree. C. to extract extractable cnstituents, separating the extract from the extractant and recycling the extractant, wherein at the extraction temperature the extractant is above its critical temperature by not more than 200.degree. C. The extraction bed may comprise a fluidized bed system.
Amirijafari et al., in U.S. Pat. No. 4,495,057, disclose a combination thermal and solvent extraction oil recovery process. Simultaneously operating thermal and solvent extraction operations are arranged in parallel, and each receives a stream of tar sand crushed to a different fineness. Heat from the spent sand is used to raise the temperature of the sand and relatively heavy oil-solvent mixture produced in the solvent extraction operation and to recover solvent from the sand and oil products taken from the extraction operation to increase the overall energy efficiency of the operation. Lighter oil from the thermal operation is blended with the heavier oil product from the solvent extraction operation. The oil product from the thermal operation is suitable for providing a make-up solvent in the solvent extraction operation, which is recycled in a closed, solvent-circulating system back to the solvent extraction unit.
The tire industry produces and sells about 240 million tires annually. Of all of the tires removed from vehicles, about 30% are consumed in various ways: used tire market, recap market, rubber reclaim market, found rubber, and other uses. The other 70% are dumped in landfills or junk yards. Scrap tires used as landfill have no economic value and present health, safety, environmental, and handling problems. Tires do not biochemically degrade sufficiently when buried, and may resurface in landfills, providing an excellent breeding ground for vermin and mosquitoes.
Accidental fires in some landfills have polluted both air and groundwater resources, and fires are different and expensive to extinguish. For this reason, landfills may charge per tire for disposal. To avoid disposal in landfills, many metropolitan areas have accumulated large stockpiles of scrap tires, and these stockpiles are maintained by charging a collection fee similar to the disposal fee.
A number of alternatives exist to disposal of waste tires. Worn tires can be sold as used tires or recapped for continued use as a tire. Worn tires can be used as artificial reefs, highway crash barriers, or children's swings. Splitting, grinding, and rubber reclamation are other uses for worn tires. Whole or shredded tires can be burned directly or pyrolyzed for energy recovery.
Since a single tire contains about 300,000 Btu's of energy, the dumped tires represent about 5.times.10.sup.13 Btu of usable energy annually, a sizable resource.
Heretofore, the economics of tire pyrolysis appear marginal, at best, except in a few specific instances: where high tire disposal costs, low tire acquisition costs, and significant on-site energy savings can be realized; or where higher value products such as benzenetoluene are refined from the pyrolytic oil.
Although several technologies, including thermal and chemical degradation of tire rubber, have been used for many years, there is still much progress to be made in this area. Other technologies that are currently being researched are microwave devulcanization and microbiological degradation.
The scrap tire problem has been studied considerably because each tire represents a significant quantity of energy, not only in terms of heat recoverable through direct combustion, but also in terms of the energy consumed in processing petroleum and natural gas into the principal constituents of tires--carbon black, extender oil, and elastomer. Recovery of part of this energy content in a form with the highest possible value, i.e., crude oil or chemical feedstock, would provide a valuable resource and also alleviate a growing environmental problem.
Reclamation of waste rubber by microwave treatment is the subject of U.S. Pat. Nos. 4,250,158 and 4,284,616. Only fair success has been obtained with waste tires as feed after the metal and tire fabric were removed.
Of the currently feasible processes for recovering the energy of waste tires, combustion and pyrolysis appear to be two processes that can be employed on a large enough scale to have an important impact on the problem in the short term. The decision between pyrolysis and combustion has heretofore depended upon the required end use. For example, tire combustion is better suited to the production of process heat, since combustion directly releases about 75% of the combustion energy in the tire. A comparable overall combustion efficiency for pyrolysis is about 66%.
Direct combustion and asphalt modifiers are the simplest. Scrap tires have a heating value of about 15,000 Btu/lb, but the scrap tires contain about 2.5% volume sulfur, which reduces their economic value as fuel. When finely shredded, the scrap tires improve the elasticity of asphaltic pavements, but the cost of finely shredding steel-belted tires are too high. The current use of shredded tires as an asphalt filler is limited to only the tire tread that is removed in normal scrapping operations.
Another potential alternative for scrap tire disposal is microbiological conversion. Nickerson, at the Institute of Microbiology at Rutgers University and the Firestone Tire and Rubber Company have reported the results of studies of fermentation of scrap tire vulcanizates. Natural rubber has been known to be attacked by microorganisms as early as 1914, and definite indications of synthetic rubber degradation were noted in their experiments.
A highly desirable point of attack for microorganisms would be the sulfur-carbon bonds created by vulcanization. If the chemically combined sulfur could be removed without significant depolymerization, the resulting product would be a superior grade of reclaimed rubber, a relatively high-value product. Although no reported work appears to exist, specific species of mircoorganisms could be isolated that would metabolize the tire matrix and produce a valuable chemical byproduct, i.e., organic acids, fuel, or monomers.
A chemical or radiation pretreatment could accelerate the microbial digestion of tire polymeric material by random cleaving of the large highly reduced molecules. Increased kinetics of degradation would benefit the economic feasibility of any such process.
In U.S. Pat. No. 4,384,151, there is described a process and apparatus for converting waste tires to fuels. Whole tires are treated by means of heavy oil flowing thereon, without immersion in a bath of oil. Fuel oil is used for this process, and there is no recycling of heavy oil. Carbon black is not recovered from this process.
Pneumatic tires contain the following components: vulcanized rubber, a rubberized fabric containing reinforcing textile cords, steel or fabric belts, and steel-wire-reinforced rubber beads. The tires are constructed on a mold. The first layer is rubber, followed by the two beads and a number of plies of the fabric, followed by another layer of rubber, with a thick circumferential layer of tread rubber. The assembly is cured by heat in a mold that contains the tread pattern and the information embossed on the sidewalls of the tire. Most modern tires are of belted radial construction, wherein the fabric cords are oriented radially and a circumferential steel, fiberglass, or fabric belt overlays the cords and underlays the tread rubber.
The most commonly used tire rubber is styrene-butadiene copolymer (SBR), which contains about 25% by weight of styrene. In combination with SBR, other elastomers such as natural rubber (cis-polyisoprene), synthetic cis-polyisoprene, and cis-polybutadiene, are also used in tires in varying amounts. A typical recipe for tire rubber is as follows:
______________________________________ Component Wt % ______________________________________ SBR 62.1 Carbon black 31.0 Extender oil 1.9 Zinc oxide 1.9 Stearic acid 1.2 Sulfur 1.1 Accelerator 0.7 ______________________________________
The carbon black acts primarily to strengthen and impart abrasion resistance to the rubber. The extender oil is usually a mixture of aromatic hydrocarbons having the primary function of softening the rubber to make it more workable. The sulfur molecules react with the double bonds in adjacent polymer chains to cause cross-linking, which hardens the rubber and prevents excessive deformation at elevated temperatures. The accelerator acts as a catalyst for the vulcanization process and is typically an organosulfur compound such as 2-mercaptobenzothiazole. The zinc oxide and stearic acid, in addition to enhancing the physical properties of the rubber, also act in harmony with the accelerator to control the vulcanization process.
The marketability of tires depends on two dominant characteristics, tread life and traction. To some extent, these characteristics are incompatible, since all other things being equal, the softer the tread rubber, the better the traction, but the worse the tread life, and vice versa. Nevertheless, tire manufacturers can vary several parameters, such as tread depth, amount and quality of the carbon black, extent of vulcanization, amount of extender oil, relative amounts of different elastomers, and other known only to individual tire manufacturers, to achieve the best combination of tread life and traction. With so many variables, it is impossible to know the exact composition of a particular used tire, so that a complete knowledge of the mechanism of tire pyrolysis is not available.
Two characteristics that all vulcanizable elastomers have in common are the presence of double bonds in the molecular chains, and a preferred location for thermal rupture of the carbon-to-carbon bonds. The double bond is the characteristic that allows vulcanization to take place, since sulfur reacts and forms a bond between double bonds of adjacent rubber molecules. It is this cross-linking between the molecular chains of elastomer molecules at a controlled number of locations that is responsible for the property of elastomers to regain their shape after deformation. The presence of the double bond also directs the thermal rupture to the beta location relative to the double bond, i.e., the second carbon-carbon bond from the double bond. When chain rupture propagates along the chain, highly reactive free radicals are formed. The free radicals will tend to be subchains of the original elastomer molecule, and when the process is carried to its logical conclusion, the monomer or monomers from which the elastomer was formed should be produced in significant yield. Since the predominant monomers in worn tires are styrene and butadiene, these are found in the liquid products of pyrolysis. A wide variety of olefins is also produced by thermal cracking. Formation of benzene and toluene can be expected through reactions involving the styrene monomer, along with a wide range of higher aromatics and condensed ring compounds. The temperature and residence time of pyrolysis are important in determining the extent to which high molecular weight compounds are cracked; hence, higher pyrolysis temperatures and longer vapor residence times promote gas production at the expense of the liquid reaction. The solid fraction, which contains zinc oxide or zinc, steel, iron oxide, potentially a number of trace metal, carbon black, and a solid hydrocarbon residue, contains relatively less hydrocarbon residue when the pyrolysis temperature of the solids residence time is increased.
A number of criteria can be used to classify the numerous pyrolysis processes. Among these criteria are the atmosphere within the reactor, the method of heat addition, the reactor type, the process conditions, the required feed preparation, and whether the reactor is batch or continuous. The most important of these criteria is the atmosphere within the reactor, i.e., whether the atmosphere is oxidative or reductive with respect to the tire materials.
Oxidative processes include those that inject air, oxygen, or steam as reactants. Air and oxygen injection result in the combustion of portion of the tire materials to give carbon monoxide, carbon dioxide, and hydrogen, which gives rise to the term "substoichiometric combustion." The relative yield of gas is higher and the heating value of the gas is lower in oxidative processes than in reductive processes. Furthermore, the heat of pyrolysis is furnished by combustion of tire materials, so that the gas evolved need not be burned to heat the reactor. When air rather than oxygen is injected, the nitrogen in the air further degrades the heating value of the gas.
Steam is oxidative with respect to the tire materials. The predominant reactions involve the cracking of hydrocarbons to carbon monoxide, carbon dioxide, and hydrogen, so that a higher value gas product is produced than is the case for a substoichiometric combustion process using air or oxygen as the oxidizer. In contrast to air or oxygen injection, steam injection requires an external source of heat to furnish the heat of reaction for cracking, and this is typically supplied by burning all or a portion of the product gas.
The majority of pyrolysis processes are reductive. Reductive processes include those with hydrogen injection and those that produce a reductive atmosphere by excluding air and other oxidizers. The main effect of adding hydrogen is to desulfurize the tires, thus adding hydrogen sulfide to the gas and reducing the sulfur content of the oil and char. The gas from all reductive processes has a high heating value, in some cases double that of natural gas, and the usual practice is to burn a portion of the gas to heat the reactor.
As scrap tires are received for processing, they are generally shredded into 2- to 6-inch pieces. The shredding process allows some steel to be magnetically separated, but few operators remove steel at this stage of the process. Any of a number of solids transport devices can be used for moving the tire pieces into feed storage, which is typically a hopper that feeds the reactor by gravity through a multiple rotary valve sealing arrangement. Some systems feed whole tires to the reactor, which eliminates the shredder.
Pyrolysis of scrap tires produces gas, liquid, and solid products in varying proportions, depending mainly on pyrolysis temperature. Most of the processes use a portion of the gas product as a heat source for the process. Although the remainder of the gas has a high heating value, it probably cannot be marketed as pipeline gas because of excessive carbon monoxide content. Typically, the gas contains low-molecular-weight paraffins and olefins, hydrogen, carbon monoxide, and hydrogen sulfide. Consequently, this gas could be a feedstock for a number of syntheses or even for carbon black production if the hydrogen sulfide were removed and if some of all of the components were separated and purified.
The division between gas and liquid products is process dependent, depending upon the condensation temperature used to separate liquid from gas. A workable definition of gas product is that it contains most of the hydrocarbons having a carbon number of five and lower. Similarly, the liquid product is specified to contain most of the hydrocarbons having a carbon number six or higher.
The liquid product consists almost entirely of aromatic hydrocarbons, with about 26% by weight of either benzene or toluene. The balance consists of higher molecular weight aromatics. It is conceivable that benzene and toluene could be separated from the liquid product with sufficient purity to be petrochemical feedstocks, but the most likely market for the benzene and toluene content is as a high octane gasoline blending stock. The heavy oil fraction (the portion of the liquid product that remains after the benzene and toluene have been removed) can be used as an extender oil for tire rubber, or it could be catalytically cracked to yield more benzene, toluene, and xylenes for gasoline blending. Another use for this fraction is as a liquid fuel comparable to No. 6 fuel oil.
The solid product is essentially carbon, ash, sulfur, and relatively nonvolatile hydrocarbons, and it is usually referred to as "char." The carbon black may be recovered in essentially its original form.
Carbon black is essentially particulate amorphous carbon: it is produced primarily by the furnace process, wherein hydrocarbon-air mixtures are partially burned to give carbon black and combustion products. The carbon black is recovered by electrostatic precipitation and cyclones. Although other properties of carbon black, such as surface area, particle shape, purity, and the like, influence its marketability as an ingredient for tire building and other uses, the most important characteristic for a furnace black appears to be particle size. The finer the particles, the better the rubber-reinforcing properties, with the lowest grade designated as SRF (semireinforcing furnace) and the highest grade SAF (super abrasion furnace).
The quantity of waste motor oil removed from vehicles is increasing every year in many countries. Because of reduced energy prices, little economic incentive presently exists to reprocess the waste motor oil. Therefore, the waste motor oil prevents serious disposal problems.
Waste motor oil cannot be used as a boiler fuel because of its high metal content. Refining the waste motor oil is not economical, largely because of the high cost of transporting waste oils to a refining facility and too low a volume of waste oil production in individual metropolitan areas.
Pyrolysis of scrap tires to produce oil, gas, and carbon black has previously been tested in both laboratory and pilot-scale equipment. However, this research has focused on older technologies which operate at nearly isothermal conditions. In these processes, higher oil yields are obtained at low pyrolysis temperatures, but the carbon black quality is better at higher temperatures. In addition to this poor economic tradeoff in product yields and quality, these processes require relatively fine shredding of the tires and high capital investments for small-scale operations.
It can be seen from the above that tire pyrolysis is a technologically effective method of reclaiming some energy, some petrochemical products, and other products from the large numbers of tires stockpiled or discarded in landfills as wells as from waste motor oil.